The overarching goals of the proposed project are to identify shared and distinct early behavioral indicators for autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) symptoms, and to better understand early developmental pathways to ASD and ADHD in children at risk for each syndrome. ASD and ADHD are two prevalent disorders emerging early in childhood, each conferring significant long-term impairment. Co-occurrence is common, and there is evidence of shared heritability and familial transmission, as well as overlapping neural and neurocognitive abnormalities, but little i known about unique and shared features very early in development, prior to diagnosis. It is clear that determining early signs uniquely associated with the development of ASD or ADHD is critical to enhancing accurate early detection efforts and identifying ideal targets and time point for intervention. What has been less well appreciated is that the impact of prevention and early intervention efforts will also be heightened by identifying transdiagnostic factors, or processes shared across disorders, that underlie symptom development. This is an ideal approach to apply to the study of ASD and ADHD because, given evidence of shared biological and behavioral abnormalities, it is likely that some early behavioral indicators also overlap, serving as general indices of atypical development that could be leveraged for transdiagnostic treatment development and prevention efforts. Two longitudinal studies are proposed to examine early behavioral indicators and developmental trajectories in three groups of children: infant siblings of children with ASD (high-risk ASD), infant siblings of children with ADHD (high-risk ADHD), and infant siblings of typically developing children (low-risk). The K99 study aims to identify early shared and distinct predictors of the development of dimensionally-measured ASD versus ADHD symptoms at 24 months of age across key cognitive, language, social, and behavioral domains at 12 and 18 months of age. The R00 study will build upon the K99 study, following an independent sample of infants through 36 months and incorporating additional measurement strategies and categorical diagnostic outcomes, allowing for the examination of shared and distinct early developmental trajectories of children with ASD versus ADHD diagnostic outcomes. Within each study, multi-method, multi-informant, multidimensional evaluations will include direct behavioral assessments, eye tracking, parent-completed rating scales, and examiner- completed ratings. These studies will be critical for (a) elucidating shared versus distinct developmental pathways; (b) accurate early screening, assessment, and differential diagnosis; and (c) determining key domains and time points for the development of targeted, process-focused intervention and prevention programs that can be applied early in life across many groups of individuals with, or at risk for, various diagnoses, consistent with the NIH Research Domain Criteria (RDoC) goals.